1. Field of Invention
The present invention relates to an in-plane sensor and a method for making the in-plane sensor, in particular to an in-plane sensor and a method for making the in-plane sensor which are able to reduce the impact of the residual stress in the manufacturing process to the sensor.
2. Description of Related Art
An in-plane sensor senses the capacitance variation resulting from a change of the distance between two electrodes, and generates a corresponding signal. Such in-plane sensor may be used in, e.g., an accelerometer or a gyro sensor. Related art can be found in, e.g., U.S. Pat. Nos. 5,326,726; 5,847,280; 5,880,369; 6,877,374; and 6,892,576.
All of the above cited prior art include the same drawback, as described below. Referring to FIG. 1, such micro-electro-mechanical device typically includes one or more fixed structures 50 and one movable structure 60. The fixed structure 50 includes multiple fixed fingers 52 which are suspended but fixed to fixed columns 58. The fixed fingers 52 are interconnected with each other by the fixed columns 58 and connection parts 56. The fixed columns 58 are fixed to an underlying substrate (not shown). The movable structure 60 includes a proof mass body 62, extending fingers 64, connection parts 66 and anchors 68. The movable structure 60 (including the proof mass body 62, the extending fingers 64, and the connection parts 66) is suspended except the anchor 68 which is also fixed to the underlying substrate. When the overall device moves, the movement causes a change in the distance between the suspended portion of the movable structure 60 and the fixed structure 50, i.e., the distance between the fixed fingers 52 and the extending fingers 64 changes. As the fixed fingers 52 and the extending fingers 64 are the two electrodes of a capacitor, the capacitance of the capacitor correspondingly changes. Thus, by measuring the capacitance, the direction, speed and acceleration of the movement of the overall device can be calculated. These data can provide important information in the field applied to by the micro-electro-mechanical device.
Referring to FIG. 2 which is a cross-sectional view taken along the X-X section of FIG. 1, because both the fixed finger 52 and the extending finger 64 are suspended, they are very easily warped during semiconductor manufacturing process. In this case the overlapping area between the fixed finger 52 and the extending finger 64 significantly decreases, reducing the effective capacitance between them. Hence, it is desired to improve the structure of such micro-electro-mechanical device so that it is less impacted by the process.